Multiphoton microscopy for in vivo neural imaging Abstract Alzheimer's disease (AD) is a debilitating neurodegenerative disorder that affects millions and is poised to bankrupt our healthcare system unless effective treatments can be translated to the clinic. AD is characterized by the presence of amyloid plaques, neurofibrillary tangles, and severe neurodegeneration. However, it is also clear that a range of other more subtle alterations occur in the brain that may be upstream of these drastic neuropathological alterations. Animal models of AD, though imperfect, have provided key insights into how some of these severe and subtle alterations occur in the brain, with many having observable clinical correlates. We have pioneered the use of multiphoton microscopy in the brains of these mouse models and have successfully monitored both subtle and severe alterations. With the increasing popularity of multiphoton microscopy, many optical advances have been made that overcome some of the limitations of early versions of laser scanning microscopes. We propose to adopt and extend some of these advances that include long wavelength excitation, adaptive optics, and wavefront shaping to turn our current microscope into a state of the art instrument for advanced brain imaging. We will apply these new techniques to target the pathophysiology of altered neuronal circuits in AD mouse models, and use a variety of techniques to understand the mechanism of the hyperexcitability of these networks. We will exploit three model networks in anesthesized or awake animals; spontaneous cortical activity, imaging of visual cortex during presentation of visual stimulus, and slow cortical oscillations (0.6Hz) that may relate to human sleep rhythms and consolidation of memories. We will focus on the role of specific subtypes of inhibitory interneurons by taking advantage of new Cre recombinase mouse models and bring to bear the power of targeted optogenetics and calcium imaging to define the mechanisms of the circuit dysfunctions. This platform will then allow us to evaluate candidate therapeutics aimed at restoring the altered network activity that should ultimately translate to clinical treatments for AD.